The goal of this proposal is to study the process of heterosexual HIV transmission. Virus isolated soon after seroconversion in a study of discordant couples in Zambia was monophyletic in the envelope gene and represented a minor variant of the donor's virus population. These results suggest that a major genetic bottleneck occurs in heterosexual HIV transmission. Studies proposed here are designed to define the nature of this selection process. Previous in vitro studies have shown that changes in envelope can affect biological properties that could play crucial roles in transmission across mucosal surfaces. We will explore these properties to test our hypothesis that compared to the donor variants, the recipient envelopes from the Zambia study have biological activities that are advantageous for transmission. Specific Aim 1 will use in vitro infection methods and flow cytometry to evaluate whether the donor and recipient envelopes interact differently, with cell types thought to be important for transmission. Specific Aim 2 will use coreceptor binding and fusion assays to explore mechanistic and structural differences between the donor and recipient envelopes and relate these findings to differences in virus entry and transmissibility.